Compact cartridge hot runner nozzle

ABSTRACT

The present invention provides an electrically heated nozzle for injection molding which is insulated to prevent conduction of electricity and loss of thermal transmission to the casing, wherein at least a part of the electrical insulation comprises a layer of dielectric insulator material with an electrical resistance wire wound spirally thereabout and another dielectric insulator layer thereover. Also disclosed is a method for making such a nozzle which includes the steps of applying a first insulator layer, winding electrical resistance wire about the first insulator layer, applying a second insulator layer, and applying a casing layer thereover. The first and second insulating layers may be provided by spraying or through telescoping, self-supporting sleeves.

FIELD OF THE INVENTION

This invention relates generally to injection molding and moreparticularly to an injection molding nozzle having an integralelectrical heating element surrounded by layered dielectric insulation.

BACKGROUND OF THE INVENTION

Heaters for injection molding and hot runner applications are known inthe prior art, as demonstrated amply by the following U.S. Pat. Nos.2,991,423, 2,522,365, 2,769,201, 2,814,070, 2,875,312, 2,987,300,3,062,940, 3,550,267, 3,849,630, 3,911,251, 4,032,046, 4,403,405, 4,386,262, 4,557,685, 4,635,851, 4,644,140, 4,652,230, 4,771,164, 4,795,126,4,837,925, 4,865,535, 4,945,630, and 4,981,431.

Heaters are of course also amply known in non-injection moldingapplications, as shown for example in U.S. Pat. Nos. 2,088,586,2,378,530, 2,794,504, 4,438,322 and 4,621,251.

There are in general three types of heaters known for use in the hotrunner nozzles. The first is so-called “integral heaters” which areembedded or cast in the nozzle body. Examples of such nozzles aredisclosed in the following patents: U.S. Pat. No. 4,238,671, U.S. Pat.No. 4,386,262, U.S. Pat. No. 4,403,405 and EP 765728. The second isso-called “independent external heaters” which have their own supportand that can be removed and replaced. Essentially, in such a design,shown in FIG. 1a, the heating element H is external to the nozzle bodyN. Heating element H comprises a resistance wire W surrounded byelectrical insulating material E and is encased in a steel casing C.Examples of such nozzles are disclosed in the following patents: U.S.Pat. No. 3,553,788, U.S. Pat. No. 3,677,682, U.S. Pat. No. 3,831,004,U.S. Pat. No. 3,912,907, U.S. Pat. No. 4,588,367, U.S. Pat. No.5,360,333, U.S. Pat. No. 5,411,393, U.S. Pat. No. 5,820,900, EP 748678,EP 963829 and EP 444748. The third is so-called “attached externalheaters” which are positioned spirally around the exterior of the nozzleor the nozzle tip but cannot be removed therefrom by reason of beingbrazed or embedded in the nozzle surface. Referring to FIG. 1b, heatingelement H′ is embedded in a groove G′ in nozzle body N′. Examples ofsuch nozzles are disclosed in the following patents: U.S. Pat. No.4,557,685, U.S. Pat. No. 4,583,284, U.S. Pat. No. 4,652,230, U.S. Pat.No. 5,226,596, U.S. Pat. No. 5,235,737, U.S. Pat. No. 5,266,023, U.S.Pat. No. 5,282,735, U.S. Pat. No. 5,614,233, U.S. Pat. No. 5,704,113 andU.S. Pat. No. 5,871,786.

Electrical heaters have been also used in the design of the so-calledhot runner probes. Unlike the hot runner nozzles, the hot runner probesdo not comprise the melt channel. The probes are located inside the meltchannel of the nozzle and thus create an annular flow. The melt isheated from the inside and this heating approach is not applicable toall materials and applications. Examples of such nozzles are disclosedin the following U.S. Pat. Nos. 3,800,027 3,970,821, 4,120,086,4,373,132, 4,304,544, 4,376,244, 4,438,064, 4,492,556, 4,516,927,4,641,423, 4,643,664, 4,704,516, 4,711,625, 4,740,674, 4,795,126,4,894,197, 5,055,028, 5,225,211, 5,456,592, 5,527,177 and 5,504,304.

Injection molding nozzles having integral heaters typically haveelectrical heating elements, wound spirally around the nozzle, whichoffer an efficient response to the many critical process conditionsrequired by modern injection molding operations. There has been acontinuous effort in the prior art, however, to improve the temperatureprofile, the heating efficiency and durability of such nozzles andachieve an overall reduction in size. Most of these efforts have beenaimed at improving the means of heating the nozzle.

For example, U.S. Pat. No. 5,051,086 to Gellert discloses a heaterelement brazed onto the nozzle housing and then embedded in multiplelayers of plasma-sprayed stainless steel and alumina oxide. To avoidcracking of the ceramic layers caused by excessive thickness and thediffering thermal properties of the ceramic and the stainless steel,Gellert employs alternating thin layers of stainless steel and aluminaoxide. The heating element of Gellert is a nickel-chrome resistance wire(i.e. see W in FIGS. 1a and 1 b herein) extending centrally through arefractory powder electrical insulating material (i.e. see E in FIGS. 1aand 1 b), such as magnesium oxide, inside a steel casing (i.e. see C inFIGS. 1a and 1 b). The heating element is integrally cast in a nickelalloy by a first brazing step in a vacuum furnace, which causes thenickel alloy to flow by capillary action into the spaces around theheater element to metallurgically bond the steel casing of the elementto the nozzle body. This bonding produces very efficient and uniformheat transfer from the element to the nozzle body.

Nozzles with this type of electrical heaters, however, are often too bigto be used in small pitch gating due to the size of the insulated heaterrequired. These heaters are also generally expensive to make because ofcomplex machining required. Also, the manufacturing methods to makethese nozzle heaters are complex and therefore production is timeconsuming.

U.S. Pat. No. 5,955,120 to Deissler which discloses a hot runner nozzlewith high thermal insulation achieved by coating the electrical heaterwith layers of a thermally insulation materials (mica or ceramic) andhigh wear resistance material (titanium). Like Gellert, the heaterelement of Deissler has its own electrical insulation protection andthus can be placed in direct contact with the metallic nozzle body (seeFIG. 2 of Deissler). Also the heater element of Deissler is attached tothe nozzle by casting (brazing) a metal such as brass. Deissler is thussimilar to Gellert in that it discloses an insulated and brazed heaterelement. Again, as with Gellert, such a device requires many additionalsteps to braze and insulate the heater and is therefore time consuming.Also, as with Gellert, the use of an insulated element makes the size ofthe heated nozzle not well suited for small pitch applications.

In an attempt to reduce nozzle size, U.S. Pat. No. 5,973,296 to Julianoshows a thick film heater applied to the outside surface of an injectionnozzle. The nozzle heater comprises a dielectric film layer and aresistive thick film layer applied directly to the exterior cylindricalsurface of the nozzle by means of precision thick film printing. Thethick film is applied directly to the nozzle body, which increases thenozzle's diameter by only a minimal amount. Flexibility of heatdistribution is also obtained through the ability to apply the heater invarious patterns and is, thus, less limited than spiral designs.

There are limitations to the thick film heater, however. Thermalexpansion of the steel nozzle body during heating can cause unwantedcracking in the film layers due to the lower thermal expansion of thefilm material. This effect is particularly acute after a large number ofinjection cycles. The cracks could affect the resistive film heaterbecause it is not a continuous and homogeneous material (as is a wire),but rather the fine dried powder of the conductive ink, as disclosed inJuliano '296.

Another heated nozzle design is disclosed in U.S. Pat. No. 4,120,086 toCrandell. In one embodiment, Crandell '086 discloses an electricallyheated nozzle having an integral heater comprising a resistance wireheater disposed between two ceramic insulating layers. The Crandell '086nozzle is made by wrapping a metal nozzle body with flexible strips ofgreen (ie. unsintered) ceramic particles impregnated in heatdissipatable material, subsequently winding a resistance wire heatingelement around the wrapped green layer, wrapping a second layer of theflexible strips of green ceramic particles thereover, heat treating theassembly to bake out the heat dissipatable material and sinter theceramic particles together, and then compacting the assembly toeliminate air voids in the assembly. In U.S. Pat. No. 4,304,544, also toCrandell, the inventor further describes the flexible green ceramicstrips as comprising a body of green ceramic insulator particles whichare impregnated in a heat dissipatable binder material. In the greenstate, such strips are pliable and bendable, permitting them to bewrapped around the metal nozzle core, but when baked, the strips becomehard and the particles agglomerate into a mass.

The Crandell '086 and '544 nozzle has relatively thick ceramic layers,employs an awkward process for applying the ceramic layers and requiresadditional heat treatment steps in fabrication. Crandell '086 concedesthat the baking step is time consuming (see column 5, lines 20-25) andtherefore admits that the design is less preferable than otherembodiments disclosed in the patent which do not utilize this method.Also, as mentioned above, it is desirable to reduce nozzle size, whichis not possible with the thick ceramic strips of Crandell '086 and '544.

The use of ceramic heaters for both hot runner nozzle heaters and hotrunner probe heaters is also disclosed in U.S. Pat. No. 5,504,304 toNoguchi. Noguchi, like Juliano, uses a printing method to form anelectrical resistive wire pattern of a various pitch from a metal or acomposite paste. A ceramic heater embodiment for a nozzle probe (shownin FIG. 1 of Noguchi) is made by printing various electrical resistivepatterns shown in FIGS. 3-4 of Noguchi. Noguchi discloses a methodwhereby a mixture of insulating ceramic powder such as silicon carbide(SiC), molybdenum silicide (MoSi₂) or alumina (Al₂O₃) and siliconnitride (SiN), and electrically conductive ceramic powder such astitanium nitride (TiN) and titanium carbide (TiC) is sintered andkneaded into a paste, which is then printed in a snaking manner on theexternal surface of a cylindrical insulating ceramic body, as shown inFIG. 3 of Noguchi. The printing state is made denser in certain areasand, by so controlling the magnitude of the so-called “wire density,” atemperature gradient is given to the heater. The heater pattern can beformed using metals such as tungsten, molybdenum, gold and platinum. Aceramic heater embodiment for a hot runner nozzle is also disclosed inNoguchi (see FIG. 9 of Noguchi). This self-sustained ceramic heater isalso made by wire-printing using the same paste or metals. The heater isplaced over the nozzle body and is then sintered and kneaded into apaste comprising a mixture of insulation ceramic powder such as siliconcarbide, molybdenum silicide or alumina and conductive ceramic powdersuch as titanium nitride and titanium carbide. The paste is printed in asingle snaking line on the part where, again, the heater pattern isformed by applying temperature gradients by varying the magnitude ofwire density across the part.

Although Noguchi introduces a wire-printing method to achieve a certainheat profile along the nozzle it does not teach or show how thiswire-printing method is actually implemented. More detailed informationabout this wire-printing method is provided by the patentee's (SeikiSpear System America. Inc.) catalogue entitled “SH-1 Hot Runner Probe”(undated). According to the catalogue, the circuit pattern, whichprovides the resistance for heating, is screen printed direction onto a“green” or uncured ceramic substrate. The flexible “green” substratewith the printed circuit is wrapped around an existing ceramic tube andthe complete unit is fired and cured to produce a tubular heater. Theresistive circuit pattern is encased within the ceramic between the tubeand the substrate and has no exposure to the outside atmosphere. Thethermocouple is inserted through the center of the tubular heater andpositioned in the tip area. Thermocouple placement in the probe tipgives direct heat control at the gate. The ceramic heater unit is thenfixed outside the probe body. Thus, this Seiki Spear method of making aceramic heater body according to Noguchi including a printed-wire issimilar to the method disclosed in Crandell '086, with the exceptionthat Crandell uses a self-sustained resistance wire wound spirallyaround the Hnozzle between two “green” ceramic layers. As with Crandell,as well, an additional sintering step is required to sinter the greenceramic layers.

Accordingly, there is a need for a heated nozzle which overcomes theseand other difficulties associated with the prior art. Specifically,there is a need for a heated nozzle which is simpler to produce andyields a more compact design.

SUMMARY OF THE INVENTION

The present invention provides an injection molding nozzle which issmaller in diameter than most prior art nozzles but which does notsacrifice durability or have the increased manufacturing costs ofprevious small diameter nozzles. Further the nozzle of the presentinvention is simpler, quicker and less costly to produce than prior artnozzles and minimizes the number of overall steps required inproduction. In particular, the need for heat treating the dielectricmaterials of the heater is removed entirely, saving time, money andhassle in fabrication. Further, the apparatus of the present inventionprovides a removable and/or replaceable cartridge heater design whichoffers the advantage of low-cost repair or replacement of a low costheater component, rather than wholesale replacement of an intricatelyand precisely machined nozzle. The methods of the present inventionsimilarly provide reduced and simplified steps in manufacturing, as wellas permitting precise temperature patterns to be achieved in a nozzlemore simply than with the prior art.

In one aspect, the present invention provides an injection moldingnozzle comprising a nozzle body having an outer surface and at least onemelt channel through the body, a first insulating layer having achemical composition, the first insulating layer disposed on the nozzlebody outer surface so as to substantially cover at least a portion ofthe nozzle body, at least one wire element disposed exterior to and incontact with the first insulating layer, the at least one wire elementbeing connectable to a power supply capable of heating the wire element,a second insulating layer having a chemical composition, the secondinsulating layer disposed over the first insulating layer and the atleast one wire element, the second insulating layer substantiallycovering the at least one wire element and at least a portion of thefirst insulating layer, and wherein the chemical compositions of thefirst and second insulating layers remain substantially unchanged oncethe layers are disposed on the nozzle body.

In a second aspect, the present invention provides an injection moldingnozzle comprising a nozzle body assembly having an outer surface and atleast one melt channel through the assembly, the assembly having a coreand a surface layer disposed around the core, the surface layer formingat least a portion of the nozzle body assembly outer surface, the corebeing composed of a first metal and the surface layer being composed ofa second metal, the second metal having a higher thermal conductivitythan the first metal, a first insulating layer disposed on the nozzlebody assembly outer surface so as to substantially cover at least aportion of the outer surface, at least one wire element disposedexterior to and in contact with the first insulating layer, the at leastone wire element being connectable to a power supply capable of heatingthe wire element and a second insulating layer disposed over the firstinsulating layer and the at least one wire element, the secondinsulating layer substantially covering the at least one wire elementand at least a portion of the first insulating layer.

In a third aspect, the present invention provides an injection moldingnozzle comprising a nozzle body having an outer surface and at least onemelt channel through the body, a first insulating layer disposed on thenozzle body outer surface so as to substantially cover at least aportion of the nozzle body, at least one wire element disposed exteriorto and in contact with the first insulating layer, the at least one wireelement being connectable to a power supply capable of heating the wireelement, a second insulating layer disposed over the first insulatinglayer and the at least one wire element, the second insulating layersubstantially covering the at least one wire element and at least aportion of the first insulating layer, and wherein the first insulatinglayer is between 0.1 mm and 0.5 mm in thickness.

In a fourth aspect, the present invention provides an injection machinefor forming a molded article, the machine comprising a mold cavity, themold cavity formed between a movable mold platen and a stationary moldplaten, at least one injection molding nozzle connectable to a source ofmolten material and capable of feeding molten material from the sourceto the mold cavity through at least one melt channel therethrough, theat least one nozzle injection molding having a nozzle body having anouter surface and the at least one melt channel through the body, afirst insulating layer having a chemical composition, the firstinsulating layer disposed on the nozzle body outer surface so as tosubstantially cover at least a portion of the nozzle body, at least onewire element disposed exterior to and in contact with the firstinsulating layer, the at least one wire element being connectable to apower supply capable of heating the wire element, a second insulatinglayer having a chemical composition, the second insulating layerdisposed over the first insulating layer and the at least one wireelement, the second insulating layer substantially covering the at leastone wire element and at least a portion of the first insulating layer,and wherein the chemical compositions of the first and second insulatinglayers remain substantially unchanged once the layers are disposed onthe nozzle body.

In a fifth aspect, the present invention provides an injection mold toform an article, the mold comprising a mold half capable ofcommunication with a mold manifold, at least one injection moldingnozzle in flow communication with the mold half through at least onemelt channel, the at least one nozzle injection molding having a nozzlebody having an outer surface and the at least one melt channel throughthe body, a first insulating layer having a chemical composition, thefirst insulating layer disposed on the nozzle body outer surface so asto substantially cover at least a portion of the nozzle body, at leastone wire element disposed exterior to and in contact with the firstinsulating layer, the at least one wire element being connectable to apower supply capable of heating the wire element, a second insulatinglayer having a chemical composition, the second insulating layerdisposed over the first insulating layer and the at least one wireelement, the second insulating layer substantially covering the at leastone wire element and at least a portion of the first insulating layer,and wherein the chemical compositions of the first and second insulatinglayers remain substantially unchanged once the layers are disposed onthe nozzle body.

In a sixth aspect, the present invention provides an injection moldingnozzle comprising the steps of providing a nozzle body, the nozzle bodyhaving an outer surface and at least one melt channel through the bodyproviding a first insulating layer on the outer surface of the nozzlebody, the first insulating layer having a chemical composition, thefirst insulating layer substantially covering at least a portion of thenozzle body outer surface, positioning at least one wire elementexterior to and in contact with the first insulating layer, the at leastone wire element being connectable to a power supply capable of heatingthe at least one wire element, providing a second insulating layer onthe first insulating layer and the at least one wire element, the secondinsulating layer having a chemical composition, the second insulatinglayer substantially covering the at least one wire element and at leasta portion of the first insulating layer, and wherein the chemicalcompositions of the first and second insulating layers remainsubstantially unchanged once the layers are provided on the nozzle body.

In a seventh aspect, the present invention provides an injection moldingnozzle comprising the steps of providing a nozzle body, the nozzle bodyhaving an outer surface and at least one melt channel through the bodypositioning a self-supporting insulating sleeve around the nozzle body,the sleeve substantially covering at least a portion of the nozzle bodyouter surface positioning at least one wire element exterior to and incontact with the insulating sleeve, the at least one wire element beingconnectable to a power supply capable of heating the at least one wireelement, providing a second insulating layer on the insulating sleeveand the at least one wire element, the second insulating layersubstantially covering the at least one wire element and at least aportion of the insulating sleeve.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made byway of example to the accompanying drawings.

The drawings show articles made according to a preferred embodiment ofthe present invention, in which:

FIGS. 1a and 1 b are partial sectional views of heated nozzleconfigurations according to the prior art;

FIG. 2 is a sectional view of a portion of an injection molding systemshowing a heated nozzle according to a preferred embodiment of thepresent invention;

FIG. 3 is an enlarged sectional view of the nozzle of FIG. 2;

FIG. 4 is a further enharged and rotated (90° counter-clockwise)sectional view of the heater assembly of the nozzle of FIG. 2;

FIG. 5 is an enlarged sectional view, similar to FIG. 4, of an alternateembodiment of a nozzle heater assembly according to the presentinvention;

FIG. 6 is an enlarged sectional view, similar to FIG. 4, of anotheralternate embodiment of a nozzle heater assembly according to thepresent invention;

FIG. 7 is an enlarged sectional view, similar to FIG. 4, of a furtheralternate embodiment of a nozzle heater assembly according to thepresent invention;

FIG. 8 is an enlarged sectional view, similar to FIG. 4, of a yetfurther alternate embodiment of a nozzle heater assembly according tothe present invention;

FIG. 9 is an exploded isometric view of an alternate embodiment of thenozzle heater of the present invention;

FIG. 10 is a sectional view of a further embodiment of the nozzle heaterof the present invention;

FIG. 11 is an enlarged sectional view of another nozzle embodimentemploying a heater according to the present invention;

FIG. 12a is an isometric view of a straight wire element for use as aheater element of the present invention;

FIG. 12b is an isometric view of a coiled wire element for use as aheater element of the present invention;

FIG. 13a is an isometric view of a doubled and twisted straight wireelement for use as a heater element of the present invention; and

FIG. 13b is an isometric view of a doubled, coiled wire element for useas a heater element of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A multi-cavity injection molding system made in accordance with thepresent invention is shown in the Figures generally at M. Referring toFIG. 2, a portion of injection molding system M is shown. A melt passage10 extends from a common recessed inlet 12 in a manifold extension 14 toan elongated manifold 16 where it branches out to a number of outlets18. As can be seen, each branch 20 of melt passage 10 extends through asteel nozzle 22, having a central melt bore 24 in communication withmelt passage outlet 18 from manifold 16 to a gate 26 leading to eachcavity 28. Nozzle 22 is a heated nozzle having a heater 30 according toa preferred embodiment of the invention, as described in greater detailbelow.

Manifold 16 is heated by a heating element 32 which may be integrallybrazed into it. Manifold 16 is held in place by a central locating ring34 and insulating pressure pads 36. Locating ring 34 bridges aninsulative air space 38 between manifold 16 and a cooled spacer plate40. Pressure pads 36 provide another insulative air space 42 betweenmanifold 16 and a cooled clamp plate 44. Spacer plate 40, clamp plate 44and cavity plate 46 are cooled by pumping cooling water through aplurality of cooling conduits 48. Clamp plate 44 and spacer plate 40 aresecured in place by bolts 50 which extend into cavity plate 46. Manifoldextension 14 is held in place by screws 52 and a locating collar 54which is secured to the clamp plate 44 by screws 56.

Each nozzle 22 is seated in a well 58 in spacer plate 40. An insulativeair space 64 is provided between heated nozzle 22 and the surroundingcooled spacer plate 40.

Referring to FIGS. 2 and 3, nozzle 22 has a body 68 having a steelcentral core portion 70, an outer surface 72, and a tip 74, which isseated in gate 26. Tip 74 has a flow channel 76 which is aligned withcentral melt bore 24. Nozzle 22 is seated and secured in manifold 16 bya threaded portion 78. Heater assembly 30 has an electrical resistivewire heating element 80, having a cold pin connections 82 for connectingwire element 80 to a power supply (not shown). Heater assembly 30 alsohas a first insulating layer 84 and a second insulating layer 86disposed on either side of wire element 80, so as to “sandwich” element80 therebetween. First layer 84 is positioned on core 70, with wireelement 80 wrapped therearound, and second layer 86 positionedthereover. An outer steel layer 88 is provided to finish nozzle 22.These layers are provided in a manner as will be described in moredetail below.

Wire element 80 is a simple, bare, electrically and thermallyuninsulated wire, preferably of thirty (30) gauge chromium nickel,though any wire material having resistive heating characteristics may beemployed. Wire element 80 is preferably wrapped around nozzle 22, andmay be provided in any arrangement which provides the temperaturedistribution desired for a particular application. For example, in theembodiment of FIG. 3, successive windings of wire element 80 are closertogether at the ends of nozzle 22, where more heat is typicallyrequired, with a more spaced distribution occurring in the centralportion of nozzle 22.

According to the present invention, first layer 84 and second layer 86are dielectric materials which can be applied in a “finished” (i.e.“non-green”) state to the nozzle body. In other words, the dielectricmaterial does not require additional heat treating steps once it isapplied to the nozzle assembly, and thus has a chemical compositionwhich does not change after it is applied to the apparatus and thematerial does not require heat treating or sintering its “finished”state. In addition to this constraint, first layer 84 is also preferablya dielectric material which can withstand the high operatingtemperatures and heater wattages experienced in hot runner injectionmolding. As one skilled in the art will understand, the dielectric ispreferably a good thermal conductor with low heat capacity, acombination which encourages rapid heating (and cooling) with maximumefficiency. The dielectric should also be a good electrical insulator,since wire element is otherwise uninsulated from nozzle 22. The choiceof material depends also on the temperature target for the moltenmaterial which will flow through the melt channel of the nozzle.

Illustrative of the dielectric materials which can be used in thepractice of this invention are: aluminum oxide; magnesium oxide; micacoatings; Vespel™ (trade mark of E.I Du Pont de Nemour & Company);graphite; alumina; alumina-silica; zirconia-based materials, such astetragonal zirconia polycrystals (TZP) partially stabilised zirconia(PSZ), fully stabilised zirconia (FSZ), transformation toughenedceramics (TTC), zirconia toughened alumina (ZTA) and transformationtoughened zirconia (TTZ); Cerama-Dip™ 538N (trade mark of AremcoProducts Inc.), a zirconium silicate-filled water-based high temperaturedielectric coating for use in insulating high-power resistors, coils andheaters; and Ceramacoat™ 538N (trade mark of Aremco Products Inc.) is asilica based, high temperature dielectric coating for use in insulatinginduction heating coils. Aluminum oxide is a preferred material becauseof its relatively high thermal conductivity.

Second layer 86 is provided to protect wire element 80 from thedeleterious effects of the atmosphere, such as oxidation and corrosion,and to insulate the exterior of nozzle 22 electrically and thermally, soas to direct the output of heater assembly 30 towards the melt in flowchannel 76. Second layer 86 may be made from the same dielectricmaterial as first layer 84 or a different material. In someapplications, it may be desirable to use different materials. Forexample, the first layer 84 may be fabricated from a material havinggood electric insulating properties but high heat conductivecharacteristic, while the second layer 86 is of a material having highelectric insulating properties and high heat insulating properties, sothat the heat is directed to the central melt bore 24 within body 68,while outer layer 88 remains cooler. The use of the same material,preferably aluminum oxide, for first layer 84 and second layer 86 ispreferred.

First layer 84 and second layer 86 may be provided as particles or aliquid sprayed onto the nozzle apparatus, as a liquid “painted” onto theapparatus or as a solid, pre-fabricated, self-supporting sleeve, asdescribed in more detail below. The layers may be provided inthicknesses as desired to suit a particular application. Thicknesses ofthe layers can range from 0.1 mm to 3 mm, and thicker, depending on theamount of insulating, overall nozzle diameter and method of fabricationdesired, as will be described further below. Thicknesses in the range of0.1 mm to 0.5 mm are preferred.

Outer layer 88 may be applied by spraying or by shrink-fitting a sleeveon second layer 86. Outer layer 88 may have any desired thickness,though a thickness of about 1.5 mm is preferred.

Referring to FIGS. 4-7, other embodiments of a nozzle heater accordingto the present invention are shown. In the embodiment of FIG. 5, asecondary wire element 90 is provided around second layer 86, protectedby a third insulating layer 92. In this three-layer embodiment, secondlayer 86 is preferably a good heat conductor and electrical insulatorwhile third layer 92 is a dielectric having good thermal insulatingcharacteristics. Third layer 92 can be chosen from the same set ofmaterials as described above for layers 84 and 86. This embodimentpermits a higher wattage heater to be obtained, at the obvious expenseof a slightly larger nozzle diameter. Alternatively, secondary wireelement 80 can provide redundancy for operational use if and when theprimary wire element fails. FIG. 6 shows a configuration similar to FIG.4, but with integral temperature sensors or thermocouple wires 94 and 96positioned between first layer 84 and second layer 86, wound spirallyaround nozzle 22 adjacent wire element 80. Inclusion of thermocouples 94and 96 allow for exacting temperature control in nozzle 22, as will beunderstood by one skilled in the art. The thermocouples may be disposedimmediately adjacent wire element 80, as shown in FIG. 6, or may beprovided between second layer 86 and third insulating layer 92, asdepicted in FIG. 7. In this embodiment, second layer 86 and third layer92 preferably have similar characteristics as described above for theFIG. 5 embodiment.

Referring to FIG. 8, in a further alternate embodiment, a metal surfacelayer 98 is provided on outer surface 72, between nozzle core 70 andfirst layer 84. Surface layer 98 is a layer of a metal having a higherthermal conductivity than steel nozzle body 68, such as copper andalloys of copper. Surface layer 98 thus promotes a more evendistribution of heat from heater assembly 30 to the pressurized melt incentral melt bore 24. Surface layer 98 may be applied by spraying or byshrink-fitting a sleeve on core 70. Surface layer 98 may have athickness of between 0.1 mm to 0.5 mm, or greater if desired.

Referring to FIG. 9, in an alternate embodiment of the presentinvention, nozzle 22′ has a core 70′, a surface layer 98′ and a heaterassembly 30′, which is composed of a first layer 84′, a wire element80′, a second layer 86′ and an outer layer 88′. In this embodiment,surface layer 98′, first layer 84′, second layer 86′ and outer layer 88′are, in fact, self-supporting, substantially rigid, annular telescopingsleeve components 98 a, 84 a, 86 a, and 88 a, respectively, which arepre-fabricated, prior to assembly of nozzle 22′, according to a methodof the present invention, described below. This sleeve constructionpermits a heater assembly 30′ configuration which is selectivelyremovable in part or in whole, depending on the design, from nozzle 22′for periodic inspection, repair and/or replacement. Also, this sleeveconstruction permits the nozzle body to expand independently from theinsulating layers, by virtue of the separate and self-supporting natureof the heater sleeves. Thus, when thermal expansion occurs in thenozzle, nozzle body 68 is free to grow longitudinally while theinsulating sleeves and wire, which typically have lower thermalexpansion characteristics, will not be subject to a mechanical stressinduced by this nozzle body expansion. This feature has beneficialimplications for increased heater durability.

The self-supporting annular sleeves of this embodiment may be made ofany suitable dielectric material, as described above, that can bemachined, molded or extruded into a thin-walled tube. As with theprevious embodiments, it is desirable that the coefficient of thermaltransfer to be higher for inner sleeve than the outer sleeve. Bothsleeves are preferably made of the same materials.

Further, as one skilled in the art will appreciate, the various layersof a particular heater need not all be applied in an identical mannerbut rather a combination of layer types may be employed. One willfurther appreciate that the removability benefit of the sleeveembodiment requires that only at least one of the layers be aself-supporting sleeve, to permit it to be slidably removed from thenozzle assembly. For example, if first layer 84′ is provided as aself-supporting sleeve, second layer 86 may be applied directly to firstlayer 84 (and over wire element 80, as well) by spraying or othercoating method, as described further below. Conversely, in a particularapplication, it may be desirable to spray or otherwise coat a firstlayer 84 onto the nozzle body, and provide second layer 86 in a sleeveformat. In such a configuration, wire element 80′ may be integrallyprovided on the interior of the second layer sleeve element, so as to beremovable therewith. Other combinations of layer construction areequally possible, as described below.

Referring to FIG. 10, in an alternate nozzle embodiment, heater assembly30″ is disposed centrally within nozzle 22″. Heater 30″ has a core 70″,first layer 84″, wire element 80″, second layer 86″ and outer layer 88″.A removable nozzle tip 74″ is provided to permit heater assembly 30″ tobe removed from nozzle 22″ for inspection, repair or replacement, asdescribed above.

The present invention may be employed in any known injection moldingnozzle design. Referring to FIG. 11, a two-part nozzle configurationaccording to the present invention is shown. A forward nozzle 100 has aheater assembly 102 according to the present invention, as describedabove, and a rearward nozzle 104 has a heater 106 according to the priorart, such as, for example, as is described in U.S. Pat. No. 5,051,086 toGellert, incorporated herein by reference. Heater assembly 102 has awire element 110, a first insulating layer 112 and second insulatinglayer 114, similar to that described above.

It will be apparent to one skilled in the art that the present inventioncan be employed using a straight wire 120, as shown in FIG. 12 a, aselement 80 to be wound spirally around the nozzle body, as describedabove. Equally, however element 80 may be a coiled wire 122, as shown inFIG. 12b, spirally wound around the nozzle. “Coiled” in this.application means helical or spring-like in nature, as illustrated inFIG. 12b. Coiled wire heating elements are well-known in the heating artas allowing for a reduction in heater power for a given operatingtemperature.

Similarly, referring to FIG. 13a, it will be appreciated that the lengthof element 80 can be effectively doubled by folding over the wireelement, and optionally twisted, to create a unitary element 124.Element 124, as expected, has twice the length of wire for a givenelement 80 length, and is twice as thick. Referring to FIG. 13b, acoiled and doubled element 126 can equally be provided.

Referring again to FIG. 3, in use wire element 80 is energized by apower source (not shown). As current flows through wire element 80,resistance to the electrical flow causes the wire to heat, as is wellunderstood in the art. Heat generated by the element is preferablychannelled and expelled substantially inwardly, by the presence firstinsulating layer 84 and second layer 86, to heat the pressurized melt incentral melt bore 76. First layer 84 and second layer 86 also provideelectrical insulation to electrically isolate wire element 80 from thesurrounding metal components of the nozzle.

The uninsulated resistive wire heating element according to the presentinvention permits a cheaper heater to be obtained while permitting moreexacting temperature distribution and control through more precise andflexible positioning of the element. Unlike the prior art, complexmachining of the nozzle body and the need for integrally brazing theheating element to the nozzle body are removed, permitting savings incost and time in fabricating the nozzle. Likewise, special and complexfilm printing techniques, materials and machinery are not required.Further, and perhaps most importantly, the present invention permitssmaller diameter heated nozzle designs to be more easily achieved andmore reliably operated than is possible with the prior art.

The heated nozzles of the present invention may be fabricated accordingto the method of the present invention. In a first embodiment of thismethod, steel nozzle body 68 is provided as the substrate for sprayingfirst layer 84 thereon. First layer 84 may be provided by spraying,“painting” or otherwise coating in a thickness of between 0.1 mm and 0.5mm. While greater thicknesses are possible, little benefit is attainedby providing a thickness greater than 0.5 mm and, since it is generallydesirable to minimize nozzle diameter, greater thicknesses are nottypically preferred. First layer 84 is provided on outer surface 72 ofnozzle body 68 so as to substantially cover, and preferably completelycover, outer surface 72 over the region where wire element 80 is to belocated. After layer 84 is dry, wire element 80 is then positionedaround first layer 84, preferably by winding wire element 80 spirallyaround the exterior of the nozzle. Although any wire pattern ispossible, winding is typically preferred because, among other things, itrequires the simplest operation in automated production. With wireelement 80 around first layer 84, second layer 86 is then provided so asto substantially cover, and preferably completely cover, wire element 80and thereby sandwich and encase wire element 80 between first layer 84and second layer 86. Second layer 86 is preferably applied by spraying,“painting” or otherwise coating to a thickness of between 0.1 mm and 0.5mm (for reasons described above), though any other method of applyingsecond layer 86 may be employed, including providing a sleeve asdescribed below. Once second layer 86 is dry, metal outer layer 88 isprovided. Metal outer layer 88 may be applied in any known manner, suchas by spraying or by shrink-fitting a sleeve, with spraying beingpreferred in this embodiment to minimize the overall diameter of thenozzle. With the outer layer applied, the assembly is then typicallyswaged to compact the assembly and bring the overall nozzle diameter towithin desired dimensional tolerances.

This embodiment of the method permits smaller diameter and more durablenozzles to be obtained than is possible with the prior art. Further, themethod is advantageous over the prior art since no additional heattreating step is required, thereby simplifying manufacture.

In an alternate embodiment of the method of the present invention, firstlayer 84 is provided as a pre-fabricated, self-supporting, substantiallyrigid, annular sleeve component which is telescopically, slidablypositioned concentrically over core 70. The sleeve element may be cast,machined, molded or extruded into a thin-walled tube, and may beprovided in any desired thickness, though thicknesses in the range of1.5 mm to 2 mm are preferred to optimize thickness and durability of thesleeve component. The inside diameter of the first layer sleeve ispreferably as small as possible while still permitting a slidinginstallation over core 70, so as to minimize any air space between thetwo components. The next step is to position wire element 80 around thefirst layer sleeve and, as one skilled in the art will understand, it isnot important whether the wire element is positioned around the firstlayer sleeve prior or subsequent to the sleeve's installation on thenozzle body. In fact, an advantage of the method of this embodiment isthat the wire element can be pre-wired on the first layer sleeve priorto installation, which can offer flexibility and simplification inmanufacturing. Once wire element 80 has been provided around the firstlayer sleeve, second layer 86 is then applied to substantially cover,and preferably completely cover, wire element 80 so as to sandwich andencase wire element 80 between the first layer sleeve and second layer86. Second layer 86 may be applied as a sleeve or by spraying, with thesleeve form being preferred in this embodiment. Again, it is notimportant whether second layer 86 is applied prior or subsequent to theinstallation of the first layer sleeve on the nozzle body. Second layer86, if applied in sleeve format, is sized to fit as closely. as possibleover wire element 80 on the first layer sleeve to minimize the air spacebetween the first and second layers. A metal outer layer 88 is thenapplied to the outside of second layer 86 and may be applied by anyknown means, such as by spraying or by shrink-fitting a sleeve, withshrink-fitting a sleeve being preferred in this embodiment. Again, aswill be understood by one skilled in the art, if a second layer sleeveis used, the outer layer may be applied to the second layer sleeveeither pre- or post-installation of the second layer sleeve on the firstlayer sleeve or the nozzle assembly. With the outer layer applied, theassembly is then typically swaged to compact the assembly and bring theoverall nozzle diameter to within desired dimensional tolerances. Theassembly is then finished as required. Such finishing steps may includeproviding removable nozzle tip 74 to the nozzle assembly, if necessaryin the particular application.

This embodiment of the method permits a removable heater assembly to beachieved. The first layer sleeve and/or second layer sleeve can beselectively removed from the nozzle body for inspection and/orreplacement, if the heater is damaged or worn, without the need toreplace the entire nozzle. Further, the independent nature of the sleeveelements permits the order of assembly to be varied as necessary, forexample, by allowing the wire element to be provided on the first layersleeve prior to installation on the nozzle body. Similarly, the secondlayer may be provided on first sleeve, over the installed wire, prior toinstallation of the first layer sleeve on the nozzle body. Thisadvantage offers not only flexibility in manufacture but also permitsthe wire element to be more precisely placed on the first layer sleeve.For example, laying the wire over the sleeve and then spinning thesleeve so as to wind the wire onto the sleeve permits a preciselycontrolled pitch and pitch variation. A further advantage of the methodis that no additional heat treating step is required, therebysimplifying manufacture.

In will be understood in the previous embodiment that, if desired, wireelement 80 can equally be pre-installed in the interior of a secondlayer sleeve, rather than the outside of first layer sleeve.

In both of the above embodiments of the method of the present invention,a metal surface layer 98 of copper or other highly thermally conductivemetal may be applied with advantage to the nozzle body prior toproviding the first insulating layer, as described above with respect tothe apparatus. In one aspect, the surface layer is applied by spraying.In another aspect, the surface layer is provided by shrink-fitting asleeve onto core 70 of nozzle body 68. As described above, the surfacelayer promotes thermal transfer between heater 30 and nozzle body 68.

While the above description constitutes the preferred embodiment, itwill be appreciated that the present invention is susceptible tomodification and change without parting from the fair meaning of theproper scope of the accompanying claims.

We claim:
 1. An injection molding nozzle comprising: (a) a nozzle bodyhaving an outer surface and at least one melt channel through said body;(b) a first insulating layer disposed on said nozzle body outer surfaceso as to substantially cover at least a portion of said nozzle body; (c)at least one wire element disposed exterior to and in contact with saidfirst insulating layer, said at least one wire element being connectableto a power supply capable of heating said wire element; and (d) a secondinsulating layer disposed over said first insulating layer and said atleast one wire element, said second insulating layer substantiallycovering said at least one wire element and at least a portion of saidfirst insulating layer, wherein at least one of said first insulatinglayer and said second insulating layer is a self-supporting sleeve. 2.The injection molding nozzle of claim 1 wherein said first insulatinglayer is composed of a dielectric material.
 3. The injection moldingnozzle of claim 1 wherein said second insulating layer is composed of adielectric material.
 4. The injection molding nozzle of claim 3 whereinsaid second insulating layer is composed of a material chosen from thegroup of aluminum oxide, magnesium oxide, mica, polyimide, graphite,alumina, alumina-silica, tetragonal zirconia polycrystals (TZP),partially stabilised zirconia (PSZ), fully stabilised zirconia (FSZ),transformation toughened ceramics (TTC), zirconia toughened alumina(ZTA), transformation toughened zirconia (TTZ), zirconium silicate andsilica.
 5. The injection molding nozzle of claim 1 further comprising anouter metal layer disposed over at least a portion of said secondinsulating layer.
 6. The injection molding nozzle of claim 1 furthercomprising an additional metal surface layer disposed over at least aportion of said nozzle body outer surface to form a layer between saidnozzle body and said first insulating layer.
 7. The injection moldingnozzle of claim 6 wherein said metal surface layer is composed of ametal having higher thermal conductivity than said nozzle body.
 8. Theinjection molding nozzle of claim 1 further comprising: (e) a secondwire element disposed exterior to and in contact with said secondinsulating layer, said second wire element being connectable to a powersupply capable of heating said second wire element; and (f) a thirdinsulating layer disposed over said second insulating layer and saidsecond wire element, said third insulating layer substantially coveringsaid second wire element and at least a portion of said secondinsulating layer.
 9. The injection molding nozzle of claim 1 furthercomprising at least one thermocouple wire positioned intermediate saidfirst insulating layer and said second insulating layer.
 10. Theinjection molding nozzle of claim 1 further comprising: (e) at least onethermocouple wire disposed exterior to and in contact with said secondinsulating layer; and (f) a third insulating layer disposed over saidsecond insulating layer and said second wire element, said thirdinsulating layer substantially covering said second wire element and atleast a portion of said second insulating layer.
 11. The injectionmolding nozzle of claim 1 wherein said first insulating layer is aself-supporting sleeve.
 12. The injection molding nozzle of claim 1wherein said wire element comprises a folded, twisted wire.
 13. Theinjection molding nozzle of claim 1 wherein said second insulating layeris a self-supporting sleeve.
 14. The injection molding nozzle of claim 1wherein said self-supporting sleeve is selectively removable from saidnozzle body.
 15. The injection molding nozzle of claim 1 wherein saidsecond insulating layer sleeve is selectively removable from said firstinsulating layer sleeve.
 16. An injection molding nozzle comprising: (a)a nozzle body having an outer surface and at least one melt channelthrough said body; (b) a first insulating layer having a chemicalcomposition, said first insulating layer disposed on said nozzle bodyouter surface so as to substantially cover at least a portion of saidnozzle body; (c) at least one wire element disposed exterior to and incontact with said first insulating layer, said at least one wire elementbeing connectable to a power supply capable of heating said wireelement; and (d) a second insulating layer having a chemicalcomposition, said second insulating layer disposed over said firstinsulating layer and said at least one wire element, said secondinsulating layer substantially covering said at least one wire elementand at least a portion of said first insulating layer, wherein saidchemical compositions of said first and second insulating layers remainsubstantially unchanged once said layers are disposed on said nozzlebody, and wherein said second insulating layer is a self-supportingsleeve.
 17. The injection molding nozzle of claim 16 wherein said sleeveis selectively removable from said nozzle body.
 18. The injectionmolding nozzle of claim 16 wherein said second insulating layer sleeveis selectively removable from said first insulating layer sleeve.
 19. Aninjection molding nozzle comprising: (a) a nozzle body having an outersurface and at least one melt channel through said body; (b) a fistinsulating layer disposed on said nozzle body outer surface so as tosubstantially cover at least a portion of said nozzle body; (c) at leastone wire element disposed exterior to and in contact with said firstinsulating layer, said at least one wire element being connectable to apower supply capable of heating said wire element; and (d) a secondinsulating layer disposed over said first insulating layer and said atleast one wire element, said second insulating layer substantiallycovering said at least one wire element and at least a portion of saidfirst insulating layer, said wire element comprising a coiled wire witha series of coils wound on said nozzle body, and wherein the density ofwinding of said coils is greater adjacent each end of said nozzle bodyand comparatively less dense adjacent the central region of said nozzlebody so that said wire element may supply relatively more heat adjacentthe ends of said nozzle body and relatively less heat in the centralregion of said nozzle body.